I. Field of the Invention
This invention is related to methods, processes, and apparatus for treatment of municipal or industrial wastewater by an activated sludge system designed for effective reduction of biological oxygen demand, reduction of suspended solids, and for nitrification.
II. Description of Related Art
Wastewater is treated by a biological oxidation process. Wastewater is conventionally mixed with an activated sludge with the simultaneous introduction of oxygen in the form of the ambient air or of a gas having an oxygen concentration greater than that of the ambient air. Aerobic microorganisms which are ubiquitous in the environment are contained in the activated sludge. By use of the oxygen mixed with the wastewater, these aerobic organisms oxidize carbon compounds which are a substantial part of the organic pollutants of wastewater. These carbon compounds are decomposed or degraded by conversion by the bacteria into further bacterial substance with carbon dioxide and water as by-products. These aerobic carbon decomposing microorganisms require oxygen to sustain their metabolic function, as well as to grow and reproduce.
Ammonia is present in many wastewaters as a by product of the decomposition of proteins. Ammonia is toxic to many forms of aquatic life, hence its removal may be required to meet environmental standards. Autotrophic bacteria remove ammonia from wastewater. These autotrophic bacteria are Nitrosomonas and Nitrobacter. These organisms are known to oxidize ammonia to nitrate in an aerobic aqueous solution within a wide range of ammonia concentration levels in the aqueous solution. These autotrophic microorganisms are ubiquitous in the environment and readily develop within the sludge. The ammonia is oxidated to nitrite by the Nitrosomonas bacteria. The nitrite is oxidated to nitrate by the Nitrobacter bacteria. This process of degradation of ammonium to nitrite and then nitrate in aerobic solution is usually termed "nitrification".
The biological oxidation of carbon containing organic pollutants and the biological decomposition of ammonium into nitrate are compatible aerobic processes and may take place in the same activated sludge. However, the use of a single activated sludge for both processes presents practical problems because of the different growth rates of the carbon consuming bacteria and of the nitrifying bacteria. The aerobic carbon consuming bacteria quickly grow and reduce the carbon bearing organic pollutants much quicker than the very slow growing autotrophic nitrifying bacteria. Also, the carbon consuming bacteria can inhibit the growth of the nitrifying bacteria in the presence of a large amount of substrate. Unless there is some provision in the process for retaining the activated sludge long enough to allow the growth and maturing of the autotrophic nitrifying bacteria, then the process will not result in nitrification of the ammonia bearing wastewater. More specifically, if the activated sludge is discharged before these slow growing nitrifying bacteria have reached maturity in sufficient numbers, nitrification will not occur.
Many communities use an aerated lagoon system for reduction of the biological oxygen demand of carbon organic waste. These are simply ponds dug into the earth. Ordinarily, their walls are sloped at a three-to-one grade, that is for each unit of rise there are three units of run in the wall grade. These lagoons may be aerated by pumping air into the lagoons or by stirring the wastewater contained therein. These lagoons are usually used in areas where there is land available for their construction. They are inexpensive to build and constitute a relatively low technology approach to treatment of the biological oxygen demand. Aerated lagoons ordinarily do not provide sufficient sludge age to obtain nitrification to meet standards imposed by regulatory agencies. Unpredictable variables, such as heavy rain water run off or variations in daily flows of waste water, temperature variations, ph variations, and the like, all have impact on the ability of an aerated lagoon to achieve nitrification. Further, in most such lagoons the activated sludge retention time is too short for growth of nitrifying bacteria.
One such lagoon system is described in Low Maintenance Mechanically Simple Wastewater Treatment, by L. G Rich, McGraw Hill, 1977. There it is recommended that an aerated lagoon system can be designed as a four-cell system consisting of a first completely suspended cell followed in series by three partially suspended cells. In the first completely suspended cell, surface aerators are used to provide a power level of at least six (6) W/m.sup.3. The partially suspended cells have a power demand of one (1) W/m.sup.3. It is recommended that the aeration cells be designed as inverted truncated pyramids. The effluent pipes to all aeration cells are discharged just below the aerators. Effluent is withdrawn from the partially suspended cells at the surface without any baffling which helps avoid retention of algae that might be generated in the system. For most size combinations for a multi-cellular aerated lagoon, the hydraulic retention time ranges from 4.74 days to 8.79 days. In the warmer summer months, in systems with higher hydraulic retention time, some growth of the autotrophic nitrifying bacteria may be achieved hence, some degree of nitrification may be achieved in an unmodified multi-cellular aerated lagoon system. However, such nitrification is uncertain and highly dependent on temperature and load variation.
A variety of ways are known to increase activated sludge age for nitrification. One way of solving the problem of maintaining sufficient activated sludge age to insure proper concentration of the nitrifying bacteria is seen in U.S. Pat. No. 4,479,876, Fuchs, which uses a carrier material having large macropores which securely attach the bacteria within the macropores of the carrier materials, which are then retained in the nitrification reactor. Thus, the nitrifying bacteria will not be carried out of the reactor with the effluent or discharged from the system with excess sludge, thus insuring a continuous presence of nitrifying bacteria. Another way of building activated sludge age is through the use of a multi-stage treatment process in which a settled sludge is removed from one of the later stages of treatment and recycled to one of the earlier stages of treatment Examples of this method of building activated sludge age can be found in Barnard U.S. Pat. No. 3,964,998 and Lorenz et al U.S. Pat. No. 5,252,214. However, these conventional systems may require extra tanks of concrete or steel, extensive operator expertise, or result in a relatively low efficiency of nitrification.
Consequently, it would be an advance in the art to modify conventional aerated lagoon or stabilization pond technology, so as to achieve a high degree of nitrification, a high tolerance for variations in wastewater inflow, and while maintaining a steady outflow of treated wastewater. A steady outflow of treated wastewater reduces the impact of pollutants remaining in the discharge, including any remaining ammonium contaminants, and makes it easier to meet environmental standards which are ordinarily measured by assuming the peak outflow is maintained throughout a given period over which the environmental impact is determined. Consequently, the present invention is concerned with apparatus and processes for nitrification that are particularly suitable for modifying a multi-cell aerated lagoon to achieve nitrifying wastewater treatment where efficiency is resistent to variations in inflow, which maintains an equal outflow, and is economical to design, to implement and simple to operate. Accordingly, in this invention there is an initial treatment cell. If an existing multi-cell aerated lagoon is being modified in accordance with this invention, one of the treatment cells in the aerated lagoon may serve as the initial treatment cell in accordance with the present invention. Depending on the needs of the operator of the system, one of the cells of a multi-cell aerated lagoon could be modified by baffles or such similar construction, so that the initial treatment cell of the present invention could occupy only a portion of a cell in a multi-cell aerated lagoon system. In the present invention, this initial treatment cell is designed to receive the initial wastewater treatment flow for the beginning of the treatment process. If the initial treatment cell of this invention is placed in a conventional multi-cell aerated lagoon, then the conventional three-to-one run-to-rise wall ratio of the conventional aerated lagoon treatment cell may be utilized to achieve flow equalization. This feature can be utilized to achieve flow equalization because in an earthen basin treatment cell with a three-to-one run-to-rise wall ratio, a relatively small increase in fluid depth in the cell represents a relatively large volume of storage capacity. Thus, when the inflow into the initial treatment cell is higher than the steady rate outflow, the difference between the volume leaving the initial treatment cell as outflow and the volume entering the initial treatment cell is as inflow will be absorbed by the excess storage capacity provided by the three-to-one run-to-rise wall ratio. When the present invention is employed in a conventional multi-cell aerated lagoon system, one of these cells could be used as an alternate cell for holding overflow sewage which exceeds the capacity of the initial treatment cell. Either the entire earthen basin treatment cell could be used for this purpose, or a portion of it could be separated by baffles from the remaining portion, the baffled portion used as an overflow holding cell for the initial treatment cell. The initial treatment cell is aerated to begin the decomposition and the reduction of the carbon containing pollutants by the aerobic carbon consuming bacteria. This first cell also acts as an aerobic selector which helps prevent sludge bulking at low food to microorganism ratios. The use of this initial aerated cell also assures mixing and dilution of any toxic or inhibitory substances in the incoming wastewater.
The partially treated effluent of the initial treatment cell is transferred at a constant rate on an alternate basis to a second or third treatment cell. Because the initial treatment cell is continuously aerated, the subsequent nitrifying cells can operate at close to a steady state condition. The second and third treatment cells go through a fill/decant, aerate, and settle cycle. The second or the third treatment cell is always being decanted at the same time and at the same rate that the cell is being filled. The second and third treatment cells are not decanted while they are being aerated, but only after they are settled. This provides that the activated sludge is retained in the second and third treatment cells as long as may be necessary to build sufficient sludge age for nitrification, even though there is a constant outflow from either the second and third treatment cells equal to the inflow from the initial treatment cell. This allows the activated sludge in both the second and third treatment cells to build sufficient age for the growth of the slow growing autotrophic nitrifying bacteria. Thus, the effluent alternately decanted from the second and third treatment cells has been nitrified. This outflow may be pumped to a fourth treatment area, which may be used for final sealing of any solids (polishing) before the effluent is discharged from the wastewater treatment plant. Nitrification may be achieved in the second and third treatment cell even if the inflow and outflow are variable. The critical factor is sufficient time to allow growth of the nitrifying bacteria, which is best achieved at a steady state but can be achieved in variable or dynamic conditions. During aeration of the second or third treatment cell, mixed liquor, hence sludge, may be recycled from the second or third cell to the aerated treatment cell. The purpose of this sludge recycling is not to build sludge age, but to guard against declining nitrification efficiency in the second and third cells because of lack of food for the nitrifying bacteria. Also, sludge may be wasted from the first, second, and third cells used for nitrification. This allows the operator of the system to achieve optimal sludge age for greatest nitrification efficiencies.
When a conventional multi-cell aerated lagoon system is being modified for nitrification in accordance with the present invention, the partially treated effluent from the initial treatment cell is ordinarily transferred to a second earthen lagoon which has been modified into three separate compartments by baffles. The rate of transfer of the partially treated effluent is constant and usually at the same average rate that wastewater enters the plant for treatment. The partially treated effluent from the initial treatment cell is transferred to a first compartment which is continuously aerated. The first compartment is separated from a second and third compartment by baffles placed within the conventional earthen basin of a conventional multi-cell aerated lagoon wastewater treatment system. Partially treated effluent is transferred from the first treatment compartment alternately to either the second or third treatment compartment. Each of the second and third treatment compartments goes through a fill/decant, aerate, and settle cycle. Either of the second or third treatment compartments is always being decanted. The second and third treatment compartments are not decanted while they are being aerated, but only after they are settled. Thus, the settled sludge is retained in the second and third treatment compartments as long as may be necessary to build sufficient sludge age for nitrification. It is desirable that the sludge from the second and third side compartments be recycled to the first compartment. This can be achieved in a simple fashion by placing mechanical stirrers used for aeration of the second and third compartments close enough to the baffles separating the second and third compartments from the first compartment so that a portion of the mixed liquor produced by the mechanical stirrers is actually thrown over the baffle to the first treatment compartment. Mixed liquor, hence sludge, may also be recycled from the second and third compartments to the first compartment by other methods including pumps and channels dedicated to that purpose. While this promotes nitrification in the first treatment compartment, the primary purpose is to return autotrophic bacteria to areas of higher food concentration, thus preventing loss of autotrophic bacteria concentration in the second and third treatment compartment. The second and third treatment compartments are decanted only after they are settled. This provides that activated sludge is retained in the second and third treatment compartments as long as may be necessary to build sufficient activated sludge age for nitrification. This is achieved even though there is a constant outflow from the second and third treatment compartments equal to the inflow to the center treatment compartment. This alternating outflow from the second and third treatment compartments may be transferred to a third cell which is used for final settling of any solids (polishing) before the now nitrified effluent is discharged from the wastewater treatment plant. Sludge may be wasted from the first, second, or third compartments thus controlling sludge age in these compartments to increase nitrification efficiency. The wasted sludge is sent to another basin for further sludge treatment, if necessary, and for eventual disposal. Once the sludge pumped to this basin has settled, supernate from this basin may be returned to the system for treatment.